WEDNESDAY, Sept. 16 (HealthDay News) -- The timing and type of hormone replacement therapy women take to relieve menopausal symptoms seem to determine the degree of breast cancer risk they face, a new French study suggests.

Overall, starting estrogen-progestagen therapy soon after menopause appears to boost the risk of breast cancer, even when only used for short periods of time, according to the study, which was published in the Sept. 14 online issue of the Journal of Clinical Oncology.

Women who began the therapy in the three years after menopause and took it for two years or less had a 54% higher risk of breast cancer compared to those who never used the therapy, said study co-author Francoise Clavel-Chapelon, a researcher at the Institut Gustave Roussy in Villejuif, France.

However, they found an exception.

"This 54% increase in risk is an average, but does not seem to apply to estrogen-progesterone menopausal hormone therapy, for which no increased risk was observed when used two years or less, even close to menopause," she said. But she noted the risk may still be present, and it was simply not found in the study due to such factors as lack of statistical power.

Many types of progestagens were taken by the women in the study; the finding that progesterone may be safer needs to be confirmed, she stressed.

The new study adds to a growing body of research that is sorting out the risks and benefits of hormone replacement therapy. Until the end of the 1990s, the therapy was viewed as largely beneficial, the authors noted. But with the release of the Women's Health Initiative trial in 2002, research began to show the therapy boosted the risk for blood clots and breast cancer, and did not protect against heart disease, as originally believed.

Later on, some researchers hypothesized that the timing of therapy might affect patients' heart disease risk. The French researchers decided to see if timing might affect breast cancer risk.

They turned to a large French study in which 1,726 invasive breast cancers were diagnosed among 53,310 postmenopausal women from 1992 through 2005.

They took into account hormone use, types of hormones, when the therapy was started and how long it was taken, along with comparing users and non-users.

In addition to finding a 54% increased risk for those who took hormone therapy for two years or less within three years of the start of menopause, they also found that short-term treatment of two years or less that was started more than three years after the beginning of menopause didn't increase breast cancer risk. However, longer duration of use elevated risk, no matter when the therapy was begun.

The new study should serve as a warning, said Leslie Bernstein, director of the division of cancer etiology at the City of Hope National Medical Center in Duarte, Calif., who wrote an accompanying editorial.

"The missing piece is, 'How safe is it to take right after menopause in terms of breast cancer risk?'" Bernstein said.

The French study provides some answers, but more research is needed, she said. The regimens typically taken in France are different than those taken by most U.S. women, Bernstein noted. And French women often use skin patches, not oral pills.

Any woman who is taking hormone replacement therapy should check with her doctor to see what hormone formulation is involved if she wants to more accurately assess her breast cancer risk.

While noting that the 54% elevated risk found by the French researchers is "worrisome," Bernstein pointed to another study finding -- that once a woman stops hormone therapy, her risk over time declines to that of the general population.

"I see this paper as a warning," Bernstein said. "You want to take a conservative approach."